1. Field of the Invention
The present invention is directed to an induction system for an engine and more particularly to an improved induction system for a V-type, four-cycle outboard motor engine.
2. Description of the Related Art
As is known in the art, the configuration of the induction system of an engine is determinative of the performance of the engine. By appropriately configuring the induction system and designing its volume and the length of the intake runners, the performance of the engine can be optimized.
This design parameter, however, is often balanced with space restrictions. That is, the induction system in some applications must be configured to fit within a defined space, sometimes sacrificing the optimal induction system design for top-end engine performance. A prime example of such compromise occurs in outboard motors. In an outboard motor, the engine is contained within a cowling which together form the uppermost portion or "powerhead" of the outboard motor.
During operation of an outboard motor with a watercraft, the powerhead may be exposed above the watercraft, thus affecting the overall aerodynamics of the watercraft. The powerhead typically includes a protective cowling that fits over the engine, in order to protect the engine and enhance the aerodynamics of the outboard motor. The protective cowling is designed to be as close fitting as possible with the engine, thus minimizing the size of and hence the aerodynamic drag caused by the powerhead.
There is an additional problem in conjunction with outboard motors related to separating water, which often is present in the surrounding air about the outboard motor during use, out of the air inducted into the induction system of the engine. For this reason, outboard motor cowling systems employ various types of devices that separate water from the inducted air. These separating systems, however, obviously reduce the air flow, and accordingly, can adversely affect engine performance.
These problems become particularly acute in conjunction with the utilization of four-cycle engines with outboard motors. Four-cycle engines are desirable for use in outboard motors because they improve emission control and better performance throughout a wider range of engine speed. However, because each cylinder in a four-cycle engine fires only once for every two revolutions of the crankshaft, the specific output of a four-cycle engine is lower than a two-cycle engine. Thus, it is desirable that high performance alternatives are used to ensure that four-cycle engines remain competitive with two-cycle engines in outboard motor construction.
Induction systems have been known to include two vertically extending plenum chambers positioned within the valley of a V-type outboard motor engine. The plenum chambers of such known induction systems are connected to the combustion chambers of the engine via a plurality of intake runners. The plenum chambers are also connected to a common intake. A throttle body is disposed downstream from the inlet to control the flow of air into the plenum chambers. In this known induction system, the inlet and the throttle body are arranged above the crankcase of the engine and facing the crankshaft. An example of such induction system is disclosed in U.S. Pat. No. 5,765,519.